Metal detectors are commonly used to locate metal objects or "targets" buried in the ground or some other background environment. In general, metal detectors sense metal targets by detecting disturbances in an electromagnetic field. There are a number of ways to detect these disturbances, but typically, they are sensed by observing signals in a search coil assembly. In this context, it is important that the metal detector distinguish between signals induced by the target and the background environment.
In practice, extracting meaningful data to identify a target in the presence of a background environment is a difficult problem. Metal targets exhibit behavior that is hard to classify because it varies with the shape and composition of the target as well as the distance and orientation relative to the detector. In addition, the behavior of the background environment changes due to changes in its composition. For example, ground can have a varying degree of iron content, which has ferromagnetic properties. The ground can also include slightly conductive materials as well. Assumptions about the signals induced by the background environment can simplify the design of the metal detector, but often lead to inaccuracies in detecting and evaluating a target.
A transmit/receive, induction balanced metal detector is one type of detector commonly used to locate metal objects in the ground. The detector is referred to as "induction balanced" because it employs an induction balanced search loop to sense targets. In this type of detector, the search loop (also known as the "search head") includes a transmit and receive coil. The two coils are designed such that the mutual inductance between them is balanced or "nulled." When a signal is applied to the transmit coil in the presence of a metal object a response signal is induced in the receive coil. Both the metal object and the background material that surrounds it can contribute to the response signal, so the signal must be processed to reduce the portion of the signal induced by the background material.
When the search loop transmits and receives a signal at a single frequency, it is particularly difficult to extract meaningful information from the response signal that is useful in evaluating a metal target. "Single frequency" metal detectors transmit and receive a signal at a single frequency, and then measure the phase angle and magnitude of the received signal. These "single frequency" detectors typically measure the response signal with two phase detectors in quadrature, and then compute a resistive to reactive ratio from the outputs of the phase detectors. The user can then attempt to identify a target buried in the ground by its resistive to reactive ratio.
It is difficult to identify targets in the background environment in a single frequency detector. As the user sweeps the search loop over the ground, the motion produces changes in the received signal. Since the frequency of these changes is different for the ground than for a metal target, filters are typically used to reduce the portion of the response signal due to the ground. When filters are used, however, the user has to move the search head skillfully to identify targets in the ground. The performance of the metal detector varies with sweep speed, making it difficult to obtain a resistive to reactive ratio that is useful in discriminating among different types of targets.
Identifying a target is especially difficult where the ground produces a strong response or changes often. Moreover, targets deeper in the ground are more difficult to detect, even for more experienced users.
Another problem with using filters is that distortion can occur when a number of metal objects are near the sweep path of the detector. Filters tend to store energy due to a target for a delayed period because of a phenomena known as group delay. As the user sweeps the search head, the filter can retain energy from a first target while the search head is actually located over another. Because of this effect, the user can miss valuable targets and waste time digging in the wrong place.
The resistive to reactive ratio computed in the single frequency metal detectors described above can assist a user in discriminating among targets. To accomplish target discrimination effectively however, the user must move the search loop very skillfully over the ground. Even assuming the user can move the search head skillfully, target discrimination is difficult because many targets have similar resistive to reactive ratios.
One approach for improving on the single frequency method is to use ground exclusion balancing. The ground response can be substantially removed by adjusting a phase axis of the detector to be in quadrature with the ground response. The ground balanced phase detector axis can be derived by summing the two quadrature outputs. Ground balancing is limited in that it only removes the ground component from the phase detector axis in quadrature with the ground response. The other phase detection axis still includes a ground component. Ground exclusion balancing, however, can be used to improve a single frequency detector by using it to trigger the measurement of a resistive to reactive ratio. For example, ground exclusion balancing circuitry can be used to gate on circuitry used to measure a resistive to reactive ratio when a target is present. As a result, the resistive to reactive ratio need only be computed when a target is near the search head. This approach can improve a single frequency metal detector, but the problems of target identification still remain.
As an alternative approach, some metal detectors transmit and receive signals at two frequencies and process the received signals to detect metal objects in a background environment. These types of detectors, to the extent known to the inventor, have a variety of limitations. First, some of these detectors process signals at different frequencies for the sole purpose of distinguishing metal objects from the background environment. These detectors fail to provide target specific data to identify an unknown target.
Second, some of these detectors make improper assumptions about response signals due to the background material. The most common assumption is that the background response does not change with frequency. In many environments, this assumption is not valid. Therefore, detectors based on this assumption produce erroneous results in background environments where the background induced signal changes with frequency.